This invention relates generally to methods and apparatus for controlling an hydraulic cylinder. More particularly this invention concerns methods and apparatus for controlling an intermittently extended hydraulic cylinder.
When handling materials having non-uniform size and heterogeneous composition, it is sometimes desirable to discharge the material from a container at an essentially uniform volumetric rate. In some instances, the container may be provided with a multi-stage hydraulic cylinder which extends into the container and pushes a bulkhead or platen that, in turn, presses against the material to expel it from the container.
When a multi-stage telescopic hydraulic cylinder is employed to discharge refuse material, the foregoing problems are compounded by the fact that successive stages of the cylinder extend at different rates when supplied with a constant volume flow of hydraulic fluid. Such variant rates of extension are especially problematic when one is trying to get an essentially uniform discharge of material with respect to time. While one manner of overcoming this disadvantage is to use a variable flow pump, variable flow pumps are substantially more expensive than constant flow pumps and are therefore undesirable where they can be avoided.
With a variable flow rate pump, the hydraulic cylinder may be provided with a control that accommodates different extension rates of successive stages and allows a low, but continuous, rate of extension as the cylinder presses against the bulkhead or platen. Such a continuous rate of elongation, however, still causes an erratic discharge of the container contents due to the non-uniformity and heterogeneity of the material.
An alternative to continuous elongation is pulsed elongation for the hydraulic cylinder. During pulsed elongation, an hydraulic cylinder may be cyclically extended a relatively short length increment at relatively high speed, and then stopped for a predetermined time. Pulsed intermittent elongation of an hydraulic cylinder may be effected by a manual operator who periodically opens and closes a control valve to cause hydraulic cylinder elongation. After a specified period, the operator again pulses the control valve to extend the cylinder.
Such manual operation is ineffective to discharge uniformly sized volumes of material since it is difficult for an individual to accurately control the time during which the control valve is open. Moreover, the individual must compensate for variable extension rates of the various cylinder stages. In addition, manual methods must rely on human accuracy and attentiveness to provide consistently regular time delays between elongation intervals. Aside from the inherent variations accompanying manual operation, a fundamental disadvantage is the presence of a human operator to actuate the cylinder. The presence of a workman is disadvantageous as it necessitates a concomitant salary expense and requires a tedious repetitive task to be performed.
To provide additional flexibility in a control adapted for use with a constant flow rate pump, it is desirable to be able to adjust the time delay between elongation increments as well as to vary the length increments through which a cylinder extends. Neither of these features are, at present, known in hydraulic cylinder control apparatus. Accordingly, it is seen that a need continues to exist for a truly effective control for an hydraulic cylinder which is connected to a constant volume flow rate pump.